The present invention relates to medical diagnostic ultrasonic imaging systems, and in particular to digital transmit waveform generators adapted for such systems.
In the prior art, digital transmit beamformers are known that use a memory such as a RAM to store a sampled version of the desired transmit waveform envelope. The data stored in RAM can be a complex baseband envelope sampled at the Nyquist frequency. See Cole, U.S. Pat. No. 5,675,554, assigned to the assignee of this invention. In this case, signal processing techniques are then used to interpolate, filter, and modulate the envelope to form the desired ultrasonic transmit waveform. In some cases, multiple simultaneous transmit beams are generated in real time in a time-interleaved manner. See the above-identified Cole patent. As another alternative, the desired ultrasonic transmit waveform can be stored directly in RAM.
The methods described above require a memory size that increases linearly with the time duration of the transmit waveform. For long transmit waveforms such as coded excitation transmit pulses, the number of samples stored in memory can exceed currently available RAM sizes. For a given RAM size, the number of samples required for each transmit waveform limits the number of concurrent RAM transmit waveforms. In some cases, this can limit the number of distinct transmit waveforms per beam, or may require reloading the RAM on a line-by-line basis, which may adversely affect the frame rate.
The process of interpolating, filtering and modulating a Nyquist-sampled baseband signal can in some cases limit the final bandwidth and frequency of the ultrasonic transmit waveform. In addition, interpolating and filtering a Nyquist-sampled signal can result in spurious signals due to non-ideal filtering. This effect is especially apparent when the carrier frequency is verniered from the center of the filter pass band.
Time interleaving multiple transmit beams is hardware efficient, but it utilizes a tradeoff between the number of transmit beams, the bandwidth, and/or the center frequency. In some implementations at the highest center frequency and the highest permitted bandwidth only a single transmit beam is allowed per channel.
By way of introduction, the preferred embodiment described below calculates ultrasonic transmit waveforms by storing a set of parameters that defines both an envelope function and a modulation function for the desired ultrasonic transmit waveform, and then calculating the ultrasonic transmit waveform in real time based on the set of parameters. The envelope function is preferably a smoothly rising and falling function, such as a Gaussian function.
In this embodiment the parameters entirely define the ultrasonic transmit waveform, and for this reason the stored parameters efficiently use system memory, even for transmit waveforms of long duration.
The preferred transmit waveform generator comprises a transmit waveform calculator that calculates the waveforms in the log domain, thereby minimizing the need for multipliers. Preferably, the waveform calculator comprises a plurality of accumulators that are operative to form respective quadratic functions in real time. These quadratic functions define the respective ultrasonic transmit waveform functions in the log or linear domain.
The disclosed embodiment combines multiple transmit waveforms using combiners that comprise of plurality of inputs and outputs, and multiplexers that switch transmit waveforms from respective single or combined transmit waveform generators to desired transducer channels.
This section is intended as a brief introduction, and it is not intended to limit the scope of the following claims.